1. Field of the Invention
The present invention relates to a heat furnace, in particular a glass furnace, the charge of which is heated to a high temperature by combustion of a fuel, this furnace being filled with electrical resistors to provide complementary electrical heating.
The invention applies in general to combustion furnaces and in particular to air heating furnaces of the regenerative type such as glass furnaces.
2. Description of the Related Art
In the prior art, a heat furnace uses a fuel to heat a charge. The combustion creates pollutant products such as CO.sub.2, NOx, SOx gases and dusts. These pollutant products are measured in the fumes downstream of the furnace. There are various requirements which need to be applied when operating a furnace and the methods used to meet these requirements, including the heat output of the furnace and reduction of pollutant products, often give rise to contradictory results.
Accordingly, the thermal performance of a furnace can be improved by pre-heating combustive gases, such as air or a mixture of air and recycled gas from a system which recycles or regenerates energy from the fumes emitted by the furnace.
In practice, most glass furnaces are more specifically fitted with a heat recycling system. The fumes leaving the furnace pass through a solid heat recovery mass in the form of packing, which is heated by contact with the fumes before they are evacuated to the exterior. A recycling system of this type is also provided at the intake of the furnace. The combustive gases pass through it and are heated in this recycling system as they do so.
In effect, the furnace operates on an alternating system, whereby the combustive gases and the fumes are circulated between the intake and the outlet first in one direction and then in the opposite direction, so that the output becomes the intake and the intake the output. At each stage of operation, fumes are fed through the recycling system located at the output which therefore recovers heat whilst the recycling system located at the intake, through which the fumes were fed during the previous stage, now has the combustive gases passing through it so that these are heated as they come into contact with the mass of heat which has built up in the recycling system.
This system of pre-heating the combustive gases has a significant effect in terms of the heat output of the furnace but, unfortunately, it increases the amount of noxious nitrous oxides, NOx, produced.
There is another solution for improving the output of the furnace, which consists of supplying energy by Joule effect inside the furnace or at the charge. This method of operating a glass furnace, known as "electric boosting", improves the overall performance of the furnace and enables the emission of NOx compounds to be reduced.
Finally, in order to reduce the emission of pollutant NOx gases, another solution is to operate a partial re-circulation of the combustion gases by injecting them back into the combustive gases upstream of the main combustion zone.